Salmonella infections are a significant cause of morbidity and mortality in both developed and developing nations. There are over 2600 serovars of Salmonella that exhibit a variety of host ranges and disease manifestations, which include gastroenteritis, bacteremia, metastatic infections and paratyphoid and typhoid fevers. In all cases, infection requires adaptation of the bacterium to distinct conditions of a number of host compartments, mediated in large part by changes in the expression of virulence and metabolism genes. Understanding these adaptations during the infection cycle is important for developing new strategies for preventing and treating infection. Although knowledge is limited, regulation of the virulence and metabolism genes of this organism are intricately linked through genetic circuitry involving SirA (Salmonella invasion regulator) and CsrA (carbon storage regulator). SirA is a DNA binding transcription factor that is the response regulator of the BarA-SirA two component signal transduction system. CsrA is an RNA binding protein that regulates mRNA translation and stability. CsrA activity is regulated by small noncoding RNAs (CsrB, CsrC), which sequester the CsrA protein. In turn, the transcription of csrB and csrC is activated by SirA. Regulation by this system responds to substrates and end products of carbon metabolism, which vary within host compartments, leading to the hypothesis that the status of carbon availability in large part governs adaptive transitions during the infection cycle. In Aim 1, a combination of genomic, bioinformatic, molecular genetic and biochemical approaches will be used to define the SirA and CsrA regulons, and thereby greatly increase our understanding of the regulatory links between metabolism and virulence. In Aim 2, several complementary approaches will be used to determine precisely when and where genes of this system are expressed and active during Salmonella infection of mice. This information will be evaluated in context with the regulons (defined in Aim 1) as well as the environmental and metabolic conditions that are known to influence these regulators in vitro. SirA and CsrA orthologs are highly conserved throughout the gamma-proteobacteria and are important for disease transmission and/or virulence in every species in which they have been examined. Thus, an understanding of the conditions and stimuli affecting SirA and CsrA activities and the mechanisms by which these proteins coordinate virulence and metabolic gene expression may be applicable to a broad range of pathogens.